Polyoma virus functions expressed by the early region of the viral genome will be studied. Altered viruses lacking restriction endonuclease sites in the early region, as well as revertants of existing thermosensitive mutants, have been isolated. They, together with naturally occurring polyoma virus variants, will be characterized with respect to the type of mutational alteration, and to the effects of the alteration upon the biological properties of these viruses. The ability of these viruses to a) replicate DNA, b) stimulate host cell DNA synthesis, and c) induce cell transformation (abortive and stable) will be analyzed. The products of the viral early region (RNAs and proteins) will be characterized.